1. Field of the Invention
The present invention relates to a carrier core material for forming a resin coated carrier used when an electrostatic latent image formed by electrophotography or electrostatic printing is developed, a resin coated carrier formed from the carrier core material, a two-component developing agent for electrophotography containing the coated carrier, and an image forming method using the two-component developing agent for electrophotography.
2. Description of Related Art
An electrophotographic developing method is a method for developing a latent image formed on a photosensitive member by allowing toner particles of a developing agent to adhere to the latent image. The developing agents employable in this method are divided into two-component developing agents comprising toner particles and a carrier and mono-component developing agents using toner particles only.
As a developing method using the two-component developing agent comprising toner particles and a carrier, a cascade method was used formerly, but at the present time, a magnetic brushing method using a magnetic roll is mainly used.
The carrier in the two-component developing agent is a carrier material which is mixed and stirred with the toner particles in a development box to impart desired electric charge to the toner particles and carries the charged toner particles onto an electrostatic latent image on a surface of a photosensitive member to form a toner image.
Also after the toner image is formed, the carrier is held by a magnet, remains on the developing roll, then returns to the development box again and is mixed and stirred with new toner particles. Thus, the carrier is repeatedly used for a certain period of time.
In the two-component developing agent, the carrier has a function of imparting desired charge properties to the toner particles when mixed and stirred with the toner particles and a function of carrying the toner particles, and has good controllability in the designing of a developing agent, differently from the mono-component developing agent. Therefore, the two-component developing agent is widely used in the field of a full color developing apparatus for which high image quality is particularly required and a high-speed printing apparatus for which reliability of image retention and durability are particularly required.
In case of the two-component developing agent, accordingly, it is necessary that the desired image properties (image density, fog, white spot (attributable to carrier adhesion or carrier fly), gradation, resolution, etc.) be stably maintained without any change from the initial stage in the period of printing impression using the two-component developing agent. To meet such requirements, it naturally becomes necessary that the properties of the carrier do not vary and be stable during the period of service.
As the carrier in the two-component developing agent, an oxide-coated iron powder or a resin-coated iron powder has been conventionally used. Since such a carrier has high magnetization and good electrical conductivity, an image having good reproducibility of a solid portion can be easily obtained by the use of a two-component developing agent using the carrier.
The above-mentioned carrier, however, is heavy in its self-weight and has too high magnetization. Therefore, by the stirring stress in the development box, toner particles are fused to the surface of the iron powder carrier, that is, so-called toner spent is liable to occur. If the toner spent occurs, an effective surface area of the carrier is decreased, and thereby triboelectric effect (frictional charging power) due to the carrier and the toner particles tends to lower. In case of the resin-coated iron powder carrier, the resin material on the surface is liable to peel off by the endurance stress. Further, because the core material is electrically conductive and has low dielectric breakdown voltage, leakage of electric charge sometimes takes place. If the leakage of electric charge takes place, the electrostatic latent image formed on the photosensitive member is broken, and as a result, brush streaks are produced on the solid portion, and it becomes difficult to obtain a uniform image. For these reasons, the two-component developing agent using the oxide-coated iron powder or the resin-coated iron powder as a carrier has been less and less used.
Instead of the oxide-coated iron powder or the resin-coated iron powder, for example, a resin-coated carrier of soft ferrite, such as Cu—Zn ferrite or Ni—Zn ferrite, has been used, as described in Japanese Patent Laid-Open Publication No. 48774/1984. Because the resin-coated carrier using soft ferrite as a core material has low magnetization, a head of the developing magnetic brush can be made soft, and hence, reproducibility of vertical and horizontal lines of the resulting image becomes good. Further, because the resin-coated carrier using soft ferrite as a core material has high dielectric breakdown voltage, leakage of electric charge rarely occurs and an image of high quality can be formed.
The resin-coated carrier using soft ferrite as a core material is excellent as a carrier for the two-component developing agent as described above, but it contains, as its main component, a heavy metal such as copper or nickel. This soft ferrite is an object of regulation based on waste regulation or environmental regulation (e.g., Title 22 of the State of California, U.S.A.), and use of the soft ferrite has been avoided. Instead of the soft ferrite containing the heavy metal, eco-friendly light metal type ferrite and carrier wherein a magnetite carrier is coated with a resin have been often adopted in recent years.
Examples of such carriers include Li—Mg—Ca type ferrite described in a Japanese Patent No. 3238006, Mn—Mg—Sr type ferrite described in Japanese Patent No. 3243376 and a magnetite granulation type carrier described in Japanese Patent Laid-Open Publication No. 458/1985.
The carriers for constituting the two-component developing agents have been gradually improved with the change of times, and with the change of developing system for electrophotography, further improvement has been required for the carriers used for the two-component developing agents.
That is to say, in the conventional developing system, an analog developing system is mainly used, but recently, the developing system has been rapidly shifted to a digital developing system. Further, also in ordinary offices, networking has been promoted, and full color images have been generally employed.
Japanese Patent No. 3168377, discloses a developing system using an alternating electric field, and it is described that by virtue of the developing system using an alternating electric field, uniformity of the solid portion can be enhanced particularly in the full color machine having many image portions.
With promotion of digitalization and coloration of electrophotographs, image quality of higher level is required, and in order to obtain high resolution, the size of toner particles in the developing agent for forming an image is reduced, and recently, a toner of finely divided particles having an average particle diameter of 5 to 10 μm has been employed.
To cope with such size reduction of the toner particles and to impart desired electric charge to the toner particles by friction, a carrier having a high specific surface area becomes necessary, and the size of the carrier particle has been reduced. More specifically, spherical ferrite having an average particle diameter of 30 to 60 μm has been employed.
If the particle size of the carrier is reduced as described above, the magnetization per particle of the carrier becomes small, so that the carrier is liable to adhere to the photosensitive member. The carrier thus adhering to the photosensitive member causes a white spot on an image that is a critical defect for a high-quality image.
With regard to this point, a countermeasure that the accuracy of classification of the carrier is enhanced to decrease a content of carrier particles of small size and thereby sharpen the particle size distribution of the carrier is taken in Japanese Patent No. 3029180. However, adhesion of the carrier to the photosensitive member cannot be completely prevented only by controlling the particle size of the carrier. In particular, development of a compact and high-speed electrophotographic apparatus (i.e., digital copying machine, printer) has been rapidly promoted, but it becomes very difficult that the developing agent follows such development of the apparatus, and in the existing circumstances, a result of forming an image with no white spot has not been obtained.
There is known a developing system wherein, in order to promote a moving speed of toner particles and thereby improve uniformity of the solid and halftone portions, AC bias is superposed on DC bias when developing bias is applied to an electrostatic latent image side from a magnetic brush, i.e., a developing system using an alternating electric field. In the developing system using an alternating electric field, however, an alternating current electric field is superposed on a direct current electric field, and consequently, a strong electric field is momentarily applied to the developing agent. In such a developing system, leakage of electric charge is liable to occur, and the leakage disorders the electrostatic latent image, so that image defects such as white spots are easily produced. In the developing system using such a high electric field, further, adhesion of the carrier onto the photosensitive member, namely, carrier adhesion is apt to be induced because of injection of electric charge to the carrier.
It is known that occurrence of the carrier adhesion is attributable to the electrical resistance of the carrier, and it is also known that the carrier adhesion is caused by a relationship between magnetic force on the magnetic roll, static electricity due to the electric field and van der Waals force.
When these reasons are considered, it is necessary to use a carrier having high magnetic force in order to prevent the carrier adhesion. When an iron powder or magnetite having high magnetic force is used, the resulting carrier has high magnetic force but has low electrical resistance, and hence the carrier adhesion cannot be reduced. In order to prevent the carrier adhesion, further, it is necessary to use a carrier having high electrical resistance. When Cu—Zn ferrite or Li—Mg—Ca ferrite having high electrical resistance is used, carrier adhesion due to electrical force can be reduced, but it has low magnetic force, and hence the carrier adhesion cannot be reduced after all in case of small particle size.
In order to prevent carrier adhesion, accordingly, it becomes necessary to use a carrier having high magnetization and high electrical resistance, and it becomes necessary to satisfy both the properties at the same time.
As a method for controlling electrical resistance of a carrier, there is known a method of coating the carrier with a resin to control electrical resistance. When such a carrier is used, high electrical resistance can be maintained in a low electric field, but in a high electric field, leakage of electric charge occurs because of an influence of electrical resistance of the core material. Especially when a core material of low electrical resistance, such as an iron powder or magnetite, is used, leakage of electric charge due to an influence of electrical resistance of the core material markedly takes place. Further, even if the conventional Cu—Zn or Ni—Zn ferrite particles or the ferrite particles described in a patent document 5 (Japanese Patent No. 3168377) and a patent document 7 (Japanese Patent Laid-Open Publication No. 69131/1996), which are considered to have relatively high dielectric breakdown voltage, are used, it is difficult to obtain uniform and faithful image reproducibility in the existing circumstances.
Japanese Patent Laid-Open Publication No. 51563/1994 and Japanese Patent Laid-Open Publication No. 35231/1994 disclose that in order to perform faithful image development, it is effective to weaken magnetization of a carrier to about 30 to 150 emu/cm3, and by the use of a carrier of such weak magnetization, the magnetic brush becomes soft in the magnetic field at the development pole and an image faithful to the latent image can be obtained. It is also described that rapid rise of magnetization at 0 to 100 Oe makes it possible to strengthen magnetization at 0 to 300 Oe, and thereby carrier adhesion can be reduced with enhancing image quality. Even by the use of this method, however, strength of the magnetization is not sufficient especially in the high-speed full color machine recently used. Therefore, carrier adhesion often takes place and image defects due to white spots are produced.
Japanese Patent Laid-Open Publication No. 181744/1995 discloses that a carrier for an electrophotographic developing agent, which is obtained by surface treating the carrier core particles with a coupling agent when the surfaces of the carrier core particles are coated with a partial hydrolysis sol obtained from Zr alkoxide or the like and cured, has an extremely rigid coating layer, so that the layer does not peel off during the period of service and a stable image can be formed. In the recent compact developing apparatus suffering heavy developing stress, however, the carrier core material is exposed by peeling of the coating layer in the printing impression process, and the resistance of the carrier core material due to the exposure causes fog or change of image density. Thus, sufficient durability has not been obtained.
Japanese Patent Laid-Open Publication No. 197214/1993 discloses that a carrier coated with a polyolefin resin containing carbon black, which is obtained by contacting a surface of a carrier core material with a high-activity catalyst component consisting of at least Ti or Zr in a hydrocarbon solvent and polymerizing an olefin monomer onto the surface, is excellent in durability, resistance to spent and resistance to environment. The carrier described in this publication, however, is a coated carrier obtained by coating the surface of the carrier core material with a coating resin, and in a high-speed apparatus suffering heavy stress, the carrier does not exhibit sufficient durability because of peeling of the coating resin.